Antivirus: NO, recommended if decide to partition for Windows, or run a VM
Mac OS X has no viruses, though there is a trojan horse, which is easily removed. (I forgot where I got it, but there is a removal tool for it). In any case: there is no registry either.
I would turn on your firewall, just so no one watches you..there are keystroke loggers and the like out there, and yes, Mac OS X is hackable (it is wise to be using the internet here safely..which I'll safely assume you are). Most of all, enjoy the problemless world of Mac OS X!!
Well okay that last part is an overstatment: there a few problems, but Apple usually has an easy fix for them through Software Update.